Steam injection techniques, such as steam stimulation and steam flooding, have been used to recover immobile heavy oils and to enhance the oil recovery from older wells where the natural field pressures are too low for unassisted production. They are designed to reduce the reservoir flow resistance by reducing the viscosity of the crude.
These techniques involve injection into the well of a high temperature wet steam in cycles of thousands of cubic meters at a time. Wet steam is a mixture of water vapor and varying amount of hot liquid water, the quality of wet steam generally ranging from 35% to 80%. Because of the density difference between the two phases of the wet steam, the vapor phase preferentially enters the upper part of the well's injection interval and the liquid phase preferentially enters the lower part.
Associated with using these wet steams is the problem of permeability damage of hydrocarbon formations containing clay minerals. "Clay mineral" is a general term for minerals such as kaolinite, illite, chlorite, smectite, and mixtures thereof. Most of these minerals have a very distinctive, book-like structure made of pages of thin layers of hydrous aluminosilicates. During steam injection, the reaction of fresh water and some clay minerals behaves much like a soaking wet book: they swell, ripple, and break off. Some clay minerals swell to 600% to 1000% of their initial volume when subjected to fresh water during steam injection. This results in (1) reducing pore volume for fluid flow and (2) plugging pore channels from fines migration. The swelling of clay and the migration of clay fines severely inhibit steam injectivity into the formation. Formations that contain clay minerals are susceptible to fresh water injection that cause the clay to disperse and migrate. When fines move downstream, they tend to bridge in pore constrictions to form miniature filter-cakes throughout the pore network. This can decrease steam injectivity in the lower interval where liquid water is injected and also in the upper injection interval where vapor phase condensation takes place. In some cases, clay structural expansion may contribute to this decrease in permeability.
It is well known that clay minerals expand greatly when the interlayers are occupied by sodium ions. A sodium ion can absorb twelve or more irregularly oriented water molecules. If the interlayer sodium ions are replaced with ammonium or potassium ions, the swelling problem may be substantially reduced.
Ammonium salts have been used to control the pH of wet steam and decrease silica dissolution. But for a high-clay-content reservoir, the concentration of ammonium ions sufficient to remedy the silica dissolution problem is usually inadequate to reduce permeability damage produced by clay minerals.
U.S. Pat. No. 4,549,609 by Watkins et al, filed Aug. 15, 1984, which is hereby incorporated by reference, attempts to solve this problem. It teaches injecting an ammoniacal nitrogen-containing compound into the wet steam to reduce permeability damage caused by silica dissolution. But this patent fails to address another problem associated with ammonium salt treatment.
To treat a high-clay reservoir for preventing permeability damage caused by clay minerals, relatively large amounts of ammonium salts must be added to the steam. Much higher concentrations of ammonium ions are required to treat a clayey formation to prevent permeability damage by clay minerals than to treat a steam to reduce permeability damage by silica dissolution. These large additions of ammonium ions to feedwater can create serious problems caused by acidity generated by decomposition of the ammonium ions in the steam generator. In Watkin's process, the pH of the generator effluent residual liquid phase is monitored to determine if the process is being properly applied. On the other hand, in our process, the ammonium ion concentrations in the vapor condensate is principally monitored and the residual liquid phase pH is secondarily monitored. Accordingly, the need exists for a further improved steam injection treatment which simultaneously (1) prevents permeability damage to a hydrocarbon-containing formation which contains clay minerals, and (2) improving the steam injection rate into the formation. It is the principle object of this invention to provide such a method.